Numerous constructions of welding sleeves are known in which a resistance heating wire is placed in the sleeve and supplied with electric power to produce heat for partly melting the sleeve and connecting it to the line members to be joined. In one known construction (German Pat. No. 2,410,039), the sleeve comprises an inner body with grooves on its outer periphery and an outer body. A resistance heating wire is placed along the bottoms of the grooves in the inner body. This known welding sleeve is used to a considerable extent for joining thermoplastic line members.
The term line members includes line or pipe parts, and shaped parts and fittings, whose connections and end portions are welded by means of welding sleeves.
In order to ensure a completely satisfactory connection, it is necessary for the connections or ends of the line members to be introduced up to a stop members located within the sleeve prior to carrying out the welding process. If one of the connections is not completely inserted into the sleeve, increased heating can occur in this area. In extreme cases, the sleeve can be destroyed at this point such that a completely satisfactory connection of the line member is not ensured.
In the known welding sleeve, the inner body with the grooves for receiving the resistance heating wire is produced by an injection moulding process employing a two-part mold with an inserted mandrel. Since the injection molding point for this mold is normally in the center of the sleeve, it is impossible to avoid severe stressing of the webs for producing the grooves which are located in the immediate vicinity of the injection molding point. Thus, only a limited number of inner bodies can be produced with this mold.